1. Field of the Invention
The present invention relates to viscometers and, more particularly, to high precision reciprocating bob viscometers.
2. Description of the Prior Art
A viscometer is an instrument used to measure the viscosity of a fluid or gas. For liquids with viscosities that vary with a rate of shear, an instrument called a rheometer is used. A viscometer typically measures the fluid/gas' resistance to deformation by shear or tensile stress, while a rheometer measures relationships between deformations and stresses. The drag caused by the relative motion of a fluid/gas over a surface is a measure of viscosity of the fluid/gas.
Rheological characteristics of fluids have been the subject of various studies for many years. Measurements have long been made in laboratories to characterize fluids, whether old or newly developed. Fluid/gas' resistance to deformation by shearing is one of the measurements made in a viscometer.
Many different types of viscometers have been developed in the past. In general, either the fluid remains stationery and an object moves through the fluid, or the object is stationery and the fluid moves past the object. The drag caused by relative motion between the fluid and a surface is a measure of the viscosity of the fluid. Viscometers developed in the past, including the following:
1. U-tube
2. Falling Sphere
3. Falling Ball
4. Falling Piston
5. Oscillating Piston
6. Vibrational
7. Rotational
8. Electromagnetically Spinning Sphere
9. Bubble
10. Rectangular-slit
Each of these above-listed viscometers has certain advantages and disadvantages. This particular invention is directed towards the oscillating piston viscometer developed by Cambridge Viscosity, Inc. as described by the company's prior patents. The sensor is a measurement chamber with a magnetically moveable piston or bob. Measurements are taken after a sample is introduced into the measurement chamber where the piston/bob resides. The piston/bob is magnetically driven in an oscillatory motion within the measurement chamber with a controlled magnetic field. The fluid being tested causes a shear stress due to the piston/bob moving through the fluid. Travel time for the piston/bob is measured. The construction parameters of the piston/bob and the measurement chamber, the strength of the magnetic field and the travel distance of the piston/bob are all used to compute the viscosity according to Newton's Law of Viscosity.
The oscillating piston/bob viscometer is particularly well suited for measuring small samples of fluids to be tested in laboratory conditions. Also, the oscillating piston/bob viscometer is particularly adapted to measure high-pressure viscosity and/or high-temperature viscosity in either laboratory or process environments.
In the oscillating piston/bob viscometer, mutual inductance between a drive coil and a monitoring coil is used to determine the position of the piston/bob, which piston/bob contains ferromagnetic material. In prior oscillating pistons/bob viscometers, a comparison is made between (1) the instantaneous voltage induced into the sensing coil and (2) the predetermined fraction of the peak value that the induced voltage achieved during the current piston/bob stroke.
Also, in prior oscillating piston/bob viscometers, the driving coil would have a small AC voltage superimposed on a large DC voltage. A very small AC voltage that is induced in the sensing coil is amplified together with noise. The amplitude of this relatively noisy signal would then be used to sense the piston/bob's position and velocity as it moves through the fluid or gas. The amplified noise made the obtaining of an accurate viscosity measurement difficult.